# std::codecvt

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< cpp‎ | locale

Defined in header `<locale>` |  |
template< class InternT, class ExternT, class StateT > class codecvt; |  |

Class template `std::codecvt` encapsulates conversion of character strings, including wide and multibyte, from one encoding to another. All file I/O operations performed through std::basic_fstream<CharT> use the std::codecvt<CharT, char, std::mbstate_t> facet of the locale imbued in the stream.

Inheritance diagram

### Specializations

The standard library is guaranteed to provide the following specializations (they are required to be implemented by any locale object):

Defined in header `<locale>`
std::codecvt<char, char, std::mbstate_t> | identity conversion
std::codecvt<char16_t, char, std::mbstate_t> (since C++11)(deprecated in C++20) | conversion between UTF-16 and UTF-8
std::codecvt<char16_t, char8_t, std::mbstate_t> (since C++20)(deprecated) | conversion between UTF-16 and UTF-8
std::codecvt<char32_t, char, std::mbstate_t> (since C++11)(deprecated in C++20) | conversion between UTF-32 and UTF-8
std::codecvt<char32_t, char8_t, std::mbstate_t> (since C++20)(deprecated) | conversion between UTF-32 and UTF-8
std::codecvt<wchar_t, char, std::mbstate_t> | conversion between the system's native wide and the single-byte narrow character sets

### Nested types

Type | Definition
`intern_type` | `InternT`
`extern_type` | `ExternT`
`state_type` | `StateT`

### Data members

Member | Description
std::locale::id `id` [static] | the identifier of the facet

### Member functions

(constructor) | constructs a new `codecvt` facet (public member function)
out | invokes `do_out` (public member function)
in | invokes `do_in` (public member function)
unshift | invokes `do_unshift` (public member function)
encoding | invokes `do_encoding` (public member function)
always_noconv | invokes `do_always_noconv` (public member function)
length | invokes `do_length` (public member function)
max_length | invokes `do_max_length` (public member function)

### Protected member functions

(destructor) | destructs a `codecvt` facet (protected member function)
do_out [virtual] | converts a string from `InternT` to `ExternT`, such as when writing to file (virtual protected member function)
do_in [virtual] | converts a string from `ExternT` to `InternT`, such as when reading from file (virtual protected member function)
do_unshift [virtual] | generates the termination character sequence of `ExternT` characters for incomplete conversion (virtual protected member function)
do_encoding [virtual] | returns the number of `ExternT` characters necessary to produce one `InternT` character, if constant (virtual protected member function)
do_always_noconv [virtual] | tests if the facet encodes an identity conversion for all valid argument values (virtual protected member function)
do_length [virtual] | calculates the length of the `ExternT` string that would be consumed by conversion into given `InternT` buffer (virtual protected member function)
do_max_length [virtual] | returns the maximum number of `ExternT` characters that could be converted into a single `InternT` character (virtual protected member function)

## Inherited from std::codecvt_base

Nested type | Definition
enum result { ok, partial, error, noconv }; | Unscoped enumeration type

Enumeration constant | Definition
`ok` | conversion was completed with no error
`partial` | not all source characters were converted
`error` | encountered an invalid character
`noconv` | no conversion required, input and output types are the same

### Example

The following examples reads a UTF-8 file using a locale which implements UTF-8 conversion in codecvt<wchar_t, char, std::mbstate_t> and converts a UTF-8 string to UTF-16 using one of the standard specializations of `std::codecvt`.

Run this code

    #include <codecvt>
    #include <cstdint>
    #include <fstream>
    #include <iomanip>
    #include <iostream>
    #include <locale>
    #include <string>

    // utility wrapper to adapt locale-bound facets for wstring/wbuffer convert
    template<class Facet>
    struct deletable_facet : Facet
    {
        template<class... Args>
        deletable_facet(Args&&... args) : Facet(std::forward<Args>(args)...) {}
        ~deletable_facet() {}
    };

    int main()
    {
        // UTF-8 narrow multibyte encoding
        std::string data = reinterpret_cast<const char*>(+u8"z\u00df\u6c34\U0001f34c");
                           // or reinterpret_cast<const char*>(+u8"zß水🍌")
                           // or "\x7a\xc3\x9f\xe6\xb0\xb4\xf0\x9f\x8d\x8c"

        std::ofstream("text.txt") << data;

        // using system-supplied locale's codecvt facet
        std::wifstream fin("text.txt");
        // reading from wifstream will use codecvt<wchar_t, char, std::mbstate_t>
        // this locale's codecvt converts UTF-8 to UCS4 (on systems such as Linux)
        fin.imbue(std::locale("en_US.UTF-8"));
        std::cout << "The UTF-8 file contains the following UCS4 code units:\n" << std::hex;
        for (wchar_t c; fin >> c;)
            std::cout << "U+" << std::setw(4) << std::setfill('0')
                      << static_cast<uint32_t>(c) << ' ';

        // using standard (locale-independent) codecvt facet
        std::wstring_convert<
            deletable_facet<std::codecvt<char16_t, char, std::mbstate_t>>, char16_t> conv16;
        std::u16string str16 = conv16.from_bytes(data);

        std::cout << "\n\nThe UTF-8 file contains the following UTF-16 code units:\n"
                  << std::hex;
        for (char16_t c : str16)
            std::cout << "U+" << std::setw(4) << std::setfill('0')
                      << static_cast<uint16_t>(c) << ' ';
        std::cout << '\n';
    }

Output:

    The UTF-8 file contains the following UCS4 code units:
    U+007a U+00df U+6c34 U+1f34c

    The UTF-8 file contains the following UTF-16 code units:
    U+007a U+00df U+6c34 U+d83c U+df4c

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 3767 | C++20 | std::codecvt<char16_t, char8_t, std::mbstate_t> and std::codecvt<char32_t, char8_t, std::mbstate_t> are locale-independent | deprecated them

### See also

Character conversions | locale-defined multibyte (UTF-8, GB18030) | UTF-8 | UTF-16
--- | --- | --- | ---
UTF-16 | `mbrtoc16` / `c16rtomb` (with C11's DR488) | `codecvt`<char16_t,char,mbstate_t> `codecvt_utf8_utf16`<char16_t> `codecvt_utf8_utf16`<char32_t> `codecvt_utf8_utf16`<wchar_t> | N/A
UCS-2 | `c16rtomb` (without C11's DR488) | `codecvt_utf8`<char16_t> | `codecvt_utf16`<char16_t>
UTF-32 | `mbrtoc32` / `c32rtomb` | `codecvt`<char32_t,char,mbstate_t> `codecvt_utf8`<char32_t> | `codecvt_utf16`<char32_t>
system wchar_t: UTF-32 (non-Windows) UCS-2 (Windows) | `mbsrtowcs` / `wcsrtombs` `use_facet`<`codecvt` <wchar_t,char,mbstate_t>>(`locale`) | `codecvt_utf8`<wchar_t> | `codecvt_utf16`<wchar_t>

codecvt_base | defines character conversion errors (class)
codecvt_byname | represents the system-supplied std::codecvt for the named locale (class template)
codecvt_utf8 (C++11)(deprecated in C++17)(removed in C++26) | converts between UTF-8 and UCS-2/UCS-4 (class template)
codecvt_utf16 (C++11)(deprecated in C++17)(removed in C++26) | converts between UTF-16 and UCS-2/UCS-4 (class template)
codecvt_utf8_utf16 (C++11)(deprecated in C++17)(removed in C++26) | converts between UTF-8 and UTF-16 (class template)

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